Process for operating a heat exchanger

ABSTRACT

The invention aims at preventing the risks of deformation and breaking down of the heat exchanger which is part of a plant for the batch treatment of fluids, of the type in which, during active periods which are separated from one another by rest periods, at least one refrigerating fluid is allowed to circulate in first ducts of the exchanger, from the cold end to the hot end of the latter, and at least one calorigenic fluid is circulated in second ducts of the exchanger, from the hot end to the cold end of the latter, characterized in that, during rest periods, heat is introduced at the hot end and cold is introduced at the cold end of the exchanger so as to maintain these two ends at temperatures which are relatively close to those corresponding to the active periods, at least one of these two inputs being supplied by means of a reserve fluid of the plant.

This application is a continuation-in-part of application Ser. No.07/846,373, filed Mar. 5, 1992 now abandoned.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to heat exchangers which operate incounter-current and which are used in plants for the batch treatment offluids.

b) Description of Prior Art

These plants cause particular problems, for the following reasons.

In continuous operation, a heat exchanger operating in counter-currenthas a temperature curve which is generally linear between its cold endand its hot end.

Since this curve is bound to the temperature of the fluids which passthrough the heat exchanger and which exchange heat with one another, anysudden pause of the circulation of these fluids produce a rapidstandardisation, by conduction, of the temperatures of the exchangertowards a temperature which is substantially the average of thetemperatures from the hot end to the cold end.

The exchanger therefore undergoes rapid variations of temperature at itsends, and a major risk of deformation or breaking down appears when itis restarted, because of the thermic shocks produced by the fluidstreated.

For example, in the case of the main heat exchanger of a plant for thedistillation of air and the production of nitrogen of the type HPN (HighPurity Nitrogen), the air treated at 8 bars enters at +20° C. and iscooled to about -169° C. in counter-current to the products which exit:nitrogen, reheated from -173° C. to +15° C. and the residual gas,reheated from -180° C. to +15°C. In permanent operation, the exchangerhas a temperature which varies linearly from about -175° C. at the coldend up to +17° C. at the hot end. If the circulation of fluids issuddenly stopped, the temperature of the exchanger rapidly reaches anequilibrium at about -80°C.

SUMMARY OF INVENTION

The invention aims at preventing the risks of deformation and breakingdown of the heat exchanger when the latter is restarted.

For this purpose, it is an object of the invention to provide a processfor operating a heat exchanger which is part of a plant for the batchtreatment of fluids, of the type in which, during active periods whichare separated from one another by rest periods, at least onerefrigerating fluid is allowed to circulate in first ducts of theexchanger, from the cold end to the hot end of the latter, and at leastone calorigenic fluid is circulated in second ducts of the exchanger,from the hot end to the cold end of the latter, characterized in that,during rest periods, heat is introduced at the hot end and cold isintroduced at the cold end of the exchanger so as to maintain these twoends at temperatures which are relatively close to those correspondingto the active periods, at least one of these two inputs being suppliedby means of a reserve fluid of the plant.

According to other characteristics:

at the end of each rest period, said quantities of heat and/or cold areprogressively increased to progressively bring the temperatures of thetwo ends of the exchanger to temperatures corresponding to activeperiods;

during the active periods, when one of the two ends of the exchanger isat a temperature near room temperature, this end of the exchanger isplaced in heat exchange relationship with the outside atmosphere duringthe rest periods;

in the case of a cryogenic plant, the hot end is placed in heat exchangerelationship with the outside atmosphere by conduction, and the cold endis placed in heat exchange relationship with evaporations of a reservecryogenic liquid of the plant;

an additional quantity of heat, is introduced at the hot end during therest periods, such as by Joule effect;

said evaporations are circulated from the cold end to the hot end of theexchanger, in said second ducts of the latter, or in ducts especiallyprovided for this purpose.

It is also an object of the invention to provide a heat exchangeradapted for the operation of such process. This exchanger, of the typecomprising a cold end, a hot end, first ducts extending from the coldend to the hot end for the circulation of a refrigerating fluid, secondsducts extending from the hot end to the cold end for the circulation ofcalorigenic fluid, is characterized in that it comprises on the onehand, at a first end, heat conductive supports extending to a source ofheat, and on the other hand means to place a reserve fluid of the plantin heat exchange relationship with the other end of the exchanger.

According to other characteristics:

said means comprise ducts of the exchanger especially adapted for thecirculation of said reserve fluid, said ducts being connected to asupply of this fluid;

the heat exchanger being of the type including brazed plates, said meanscomprise a coil mounted in heat exchange relationship on each face ofthe exchanger including the end plates, this coil being connected to asupply of storage fluid;

the coil defines a heat exchange surface which is more important in thevicinity of said other end of the exchanger;

the heat conductive supports are provided with additional heating means,such as electrical resistances.

BRIEF DESCRIPTION OF DRAWINGS

The embodiments of the invention will now be described with reference tothe annexed drawing, in which:

FIG. 1 is a partial schematical perspective view of a heat exchangeraccording to the invention; and

FIG. 2 is a similar view of another embodiment of heat exchangeraccording to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 which only represents the elements which are essential tounderstanding the invention, shows a counter-current heat exchanger ofthe type including brazed aluminum plates, which is part of a plant fortreating fluids in batch, typically a plant for the distillation of air.More specifically, this example illustrates a plant for the productionof nitrogen of the type HPN.

As it is well known, an exchanger with brazed plates consists of astacking of a plurality of aluminum plates 2, vertically superposed,which are all identical, rectangular and parallel to one another. Theseplates define therebetween a number of flat ducts. Cross-bars aremounted on the edges of these plates, and suitable interruptions ofthese bars define windows for the inlet or outlet of fluids in thesegroups of selected ducts.

The inlets-outlets of fluids are carried out by means ofsemi-cylindrical boxes disposed against the faces of the exchanger whichinclude bars.

In the example under consideration, the lower end, or cold end, of theexchanger includes three boxes:

on the vertical face of the exchanger, box 3 normally constitutes theinlet of gaseous, refrigerating nitrogen, produced by the plant; thisgaseous nitrogen is introduced into box 3 via duct 4 which is providedwith a stop valve 5;

on the lower face of the exchanger, a box 6 which is normally used forthe inlet of a residual gas, which is also refrigerating, of the plant,which gas is introduced into box 6 via duct 7 provided with a stop valve8; and

on the other vertical face of the exchanger, a box 9 which is used as anoutlet for air to be distilled, after cooling, this air constituting thecalorigenic fluid of the heat exchanger and comes out of box 9 via duct10.

The exit of nitrogen and residual gas from the exchanger is carried outby means of respective outlet boxes (not illustrated) provided at theupper end or hot end, of the exchanger; similarly, the inlet for the airto be treated is carried out by means of an inlet box (not illustrated)provided for at this upper end.

In the vicinity of its hot end, the exchanger is mounted on twohorizontal supports 11 which extend to an exterior metallic sheath 12 ofthe plant whose exterior face is in contact with the outside atmosphere.These supports are heat conductive and in order to ensure a good heatexchange, they are in close contact with the respective vertical facesof the exchanger 1 including boxes 3 and 9, along the entire width ofthese faces.

The air distillation plant comprises a supply of cryogenic liquid,which, for example, is a liquid/vapor phase separator, the bottom of adistillation column or a tank of liquid. This tank has beenschematically illustrated at 13, and it will be understood hereinafterthat it consists of a tank of liquid nitrogen. A duct 14 provided with astop valve 15 goes from the upper part of this tank 13. This duct isdivided into two ducts 16, 17 respectively ending in boxes 3 and 6.

During the periods of normal operation of the plant, the counter-currentcirculation on the one hand of the two refrigerating fluids (nitrogenand residual gas), and on the other hand of the calorigenic air to betreated, maintains both ends of the exchanger 1 at predeterminedtemperatures, for example of the order of +15° C. for the hot end with atemperature gap of about 5° C. between the outgoing and ingoing fluids,and of the order of -170° to -180° C. for the cold end, with atemperature gap of about 10° C. between the ingoing and outgoing fluids.

When the production of nitrogen is interrupted, the stop valves 5 and 8are closed and valve 15 is opened. Thus, a controlled flow of coldgaseous nitrogen is sent to all the ducts of refrigerating fluids, whilea flow of heat at room temperature reaches all the ducts of theexchanger at its hot end, via supports 11.

Thus, with a very low consumption of nitrogen, it is possible tomaintain a temperature gradient relatively close to that correspondingto the normal operation of the plant, between the hot end and the coldend of the exchanger, during the rest periods of the plant. Thisexpression should be understood in a broad sense as designating atemperature gradient between a cryogenic temperature, for example of theorder of -110° C., for the cold end, and a temperature close to roomtemperature, for example of the order of +5° C., for the hot end.

It is thus possible to prevent thermal shocks when restarting the plant,and at the same time to decrease the required time for restarting toreach a normal equilibrium of the exchanger. Moreover, heat losses arereduced because of the permanent maintenance of cold conditions at thecold end of the exchanger.

As indicated in mixed line in FIG. 1, as a variant, the exchanger 1 maybe provided with additional ducts especially adapted for the circulationof evaporations from the supply 13 during periods of rest. In this case,duct 14 directly ends into an inlet box 3A, adjacent box 3, which opensinto these additional ducts.

The embodiment illustrated in FIG. 2 differs from the previous by thefollowing points.

On the one hand, at the hot end of the heat exchanger 1A, the supports11 are provided with electrical resistances 18 which enable to bring acontrolled addition of heat at this hot end, therefore maintaining thelatter at a predetermined temperature which is near room temperature.For this purpose, the electrical current is sent in these resistancesunder the control of temperature probes 19 associated with each support11.

On the other hand, the evaporations from the reserve of liquid nitrogen13 are brought by means of duct 14, no more into boxes 3 and 6 or 3A,but in added coils 20 mounted in heat exchange relationship on the twoopposite vertical faces of the exchanger containing boxes 3 and 9.

The two coils 20 are arranged in zig-zag, on the entire width of saidfaces, with a tight pitch in the cold zone of the exchanger, where thelargest cold input is required, and a progressively increasing pitchwhile going up along the exchanger, to their exit, near supports 11,which is connected to a common duct 21 for the evacuation of reheatednitrogen.

Coils 20 are fixed on the exchanger so as to be in heat contact with allthe ducts of the exchanger. This mounting may advantageously be mixedand may include a mechanical fixing means and a gluing by means of asuitable heat conductive cryogenic resin.

It should be noted that the exchanger 1 or 1A may be mounted either in aknown cold box at atmospheric pressure or in certain plants, in a spaceunder vacuum, which inter alia, is delimited by exterior wall 12.

As a variant, another way of keeping a temperature gradient in theexchanger 1A of FIG. 2 during periods when the apparatus is not inoperation, is to provide a constant electrical power at the hot end bymeans of said resistances 18, to send the evaporations from tank 13 tothe cold end of the exchanger and to control the temperature of the hotend through the flow rate of the evaporations from the tank. Thus, theevaporations from tank 13 are sent into the exchanger (valve 15 opened)when the temperature at the hot end is higher than an upper limit (suchas 10° C.), and they are stopped (valve 15 closed) when the temperatureat the hot end becomes lower than a lower limit (such as 0° C). Underthe effect of the heat flow sent to the hot end, the temperature of thehot end rises again and, when it is higher than the upper limit, theevaporations from the tank are again introduced into the exchanger.

We claim:
 1. Process for operating a heat exchanger which is part of aplant for the batch treatment of fluids, in which during active periodswhich are separated from one another by rest periods, at least onerefrigerating fluid is allowed to circulate in first ducts of theexchanger, from the cold end to the hot end of the latter, and at leastone calorigenic fluid circulates in second ducts of the exchanger, fromthe hot end to the cold end of the latter, wherein during rest periods,heat is introduced at the hot end and cold is introduced at the cold endof the exchanger so as to keep these two ends at temperatures which arerelatively close to those corresponding to the active periods, at leastone of these two inputs being supplied by means of a reserve fluid ofthe plant.
 2. Process according to claim 1, wherein at the end of eachrest period, said quantities of heat and/or cold are progressivelyincreased to progressively bring the temperatures of the two ends of theexchanger to temperatures corresponding to the active periods. 3.Process according to claim 1, in which, during the active periods, oneof the two ends of the exchanger is at a temperature near roomtemperature, wherein this end of the exchanger is placed in heatexchange relationship with outer atmosphere during the rest periods. 4.Process according to claim 1, for a cryogenic plant, wherein the hot endis placed in heat exchange relationship with outside atmosphere, byconduction, and the cold end is placed in heat exchange relationshipwith evaporations from a reserve cryogenic fluid of the plant. 5.Process according to claim 4, wherein during the rest periods, anadditional quantity of heat is brought to the hot end.
 6. Processaccording to claim 5, wherein said additional quantity of heat isconstant, and the circulation of said evaporations is carried out whenthe temperature at the hot end exceeds an upper limit and is interruptedwhen it becomes below a lower limit.
 7. Process according to claim 5,which comprises circulating said evaporations from the cold end to thehot end of the exchanger, through said second ducts of the exchanger. 8.Process according to claim 5, wherein said additional quantity of heatis provided by the Joule effect.
 9. Process according to claim 5, whichcomprises circulating said evaporations from the cold end to the hot endof the heat exchanger through ducts especially provided for thispurpose.